McDonald's newly named Chief Executive Charlie Bell underwent colorectal cancer surgery Wednesday afternoon, two weeks after Jim Cantalupo, the leader credited with the fast food giant's stunning turnaround, suddenly died of a heart attack. The company termed the surgery successful and said that Bell would remain president and CEO during a brief recovery. It did not disclose any details about the cancer.

But McDonald's investors could be in for a rough ride Thursday given the uncertainty Bell's condition poses for McDonald's and his potential successor if Bell should have to step down from running the world's largest fast-food chain.

When Cantalupo, 60, died April 19 at a franchisee convention in Orlando, Fla., McDonald's stock tumbled 1.4 percent but quickly recovered. On Wednesday, McDonald's stock closed at $27.81, up 20 cents. The shares barely budged in after-hours trading.

After Cantalupo's death, McDonald's board quickly quelled investor's fears by naming Bell, a 43-year-old Australian and chief operating officer, the same day. McDonald's has yet to name Bell's replacement.

McDonald's declined to release details about Bell's condition. "We want to respect the privacy of Charlie Bell, so we are going to stick the press release at this time," said spokesman Walt Riker.

Bell, who started his career at McDonald's flipping burgers at age 15 and rose to run McDonald's European operations, was widely seen as following Cantalupo's back-to-basics approach. It was Cantalupo who pulled McDonald's out of a downward spiral by re-focusing McDonald's on hot food, fast service and clean restaurants. Mystery shoppers were hired to grade restaurants.

McDonald's has enjoyed six months of double-digit sales gains for the first time in 30 years. The chain,with $33.1 billion in franchised and affiliated sales for 2003, forecasts 3 percent to 5percent top-line growth and 6-7 percent operating income growth starting in 2005.

Beyond succession plans, McDonald's faces other challenges, namely tough competition in the saturated fast-food sector. The chain is routinely blamed for the bulging American waistline, despite attempts by Cantalupo to slim down its high-fat menu. If the company tampers too much, it runs the risk of driving away loyal customers or infuriating franchisees.